The present invention relates to a thermal printer, and more particularly to a structure of the thermal printer and a structure of a paper detecting sensor.
The thermal printer in which characters are formed by heating selected elements of a dot matrix that is in contact with heat-sensitive paper is widely used in printers such as the facsimile receiver.
U.S. Pat. No. 5,150,130 discloses a label printer and U.S. Pat. No. 5,422,660 discloses a label printer using a transfer ribbon.
In a printer for printing bar codes on label paper, a sensor is provided for detecting the position of the label paper as disclosed in U.S. Pat. No. 4,706,096 and 4,757,329. The paper detecting sensor is mounted on a paper passage. There are two types of the paper detecting sensor. One is the transmitting type sensor and the other is the reflection type sensor. The paper detecting sensor detects the position of the label paper by detecting-a black line or notch formed in a mount or label paper.
There is also used a slide type sensor comprising a pair of members which are opposite set at a position corresponding to the notch of the recording paper.
In the conventional printer using the transfer ribbon, the paper and ribbon must be passed through a narrow gap, and then set at predetermined positions. Consequently, it is difficult to set the paper and ribbon.
On the other hand, a cutter is mounted on the printer, the paper is fed to the cutter and cut by the cutter. Thereafter, the paper is fed back to the printing position. At that time, the ribbon fed back together with the paper is liable to be loosened to cause the ribbon to be wrinkled.
The recording paper is inserted between opposite members of the paper detecting sensor. However, since the members are closely disposed at a small space of 2 or 3 mm, it is troublesome to insert the paper.
Furthermore, if the label peels off and adheres to the paper detecting sensor, the label must be removed. However, it is difficult to remove the label, because the space between opposite members of the sensor is very narrow.
An object of the present invention is to provide a paper detecting sensor which can be opened.
According to the present invention, there is provided a thermal printer comprising a housing, a chassis assembly provided with a printer mechanism and securely mounted in the housing, a head frame assembly provided with a thermal head and rotatably mounted on the chassis assembly by a shaft which is disposed in a paper feeding direction at a side of the chassis assembly, a paper sensor having a sensor frame rotatably mounted on the chassis assembly by a shaft which is disposed in the paper feeding direction at the side of the chassis assembly.
The thermal printer further comprises closing means for closing the paper sensor together with the head frame assembly, and a pair of ribbon holders rotatably mounted on the head frame assembly, the ribbon holders rotated by power transmitting means from the print mechanism for winding an inked ribbon.
The thermal printer further comprises a ribbon rewinding device for rewinding the inked ribbon about one turn when paper is fed back and heavy load applying means is provided for applying a heavy load to one of the ribbon holders which is to be rotated in a reverse direction when the head frame assembly is opened.
A one-way clutch is provided in the power transmitting means for transmitting driving force to the ribbon holders only when the ribbon holders are rotated in a normal direction.
The closing means comprises a sensor closing plate fixed to head frame assembly so as to contact with the sensor frame when closed.
The ribbon rewinding device comprises a coil spring wound by the power transmitting means when the power transmitting means is rotated in a normal direction.
The heavy load applying means comprises a coil spring provided in the power transmitting means.
The present invention further provides a paper detecting sensor for a printer comprising upper and lower sensor frames pivotally connected to each other by a knobbed shaft, spiral shafts disposed in a paper width direction and rotatably mounted on the upper and lower sensor frames, respectively, a sensor holder having a sensor and slidably engaged with each of the spiral shafts, a first screw gear securely mounted on the knobbed shaft, second and third screw gears each of which is secured to each of the spiral shafts and engaged with the first screw gear, whereby when the knobbed shaft is rotated each of the sensor holders is moved in the paper width direction.